Various means have been devised by which acoustic signals may be generated underwater. Generally these comprise pistons impacting against a diaphragm in contact with water. U.S. Pat. Specification No. 4,095,667 to Mahig and Allen describes a portable underwater signalling device. Other acoustic signal generators are described in U.S. specifications 3,433,202 to Sharp et al and 3,277,437 to Bouyoucos.
In Mahig and Allen, U.S. Pat. No. 4,095,667, the valving to drive the piston and the piston involve complex shapes and sealing arrangements.
In Sharp, U.S. Pat. No. 3,433,202, and Bouycousos, U.S. Pat. No. 3,277,437, valving is achieved externally of the device such that these devices are not useful to divers needing a small hand held acoustic generator.